Diagnostics of mixing in the upper troposphere/lower stratosphere region reveal interesting seasonal and interannual variability and illustrate how the subtropical tropopause can be identified with a region of minimum mixing, a partial barrier to the transport of air between the troposphere and stratosphere. Both the strength and the location of this barrier show substantial seasonal variability, with a stronger barrier and weaker mixing observed during winter. The interannual variability of the mixing intensity near the tropopause on the 350 K isentropic surface suggests a correlation with the phase of the El Niño‐Southern Oscillation, with weaker mixing occurring during strong El Niño years. The diagnostics are based on the analyses and reanalyses from the European Centre for Medium‐Range Weather Forecasts over the 21 year period 1979–2000 and include the exponential stretching rates of material contours and Nakamura's [1996] modified Lagrangian mean effective diffusivity/equivalent length. As well as providing information about the spatial and temporal distribution of mixing intensity, the diagnostics also indicate sensitivity to changes in the analyses/reanalyses data sets. In particular, a stronger seasonal cycle and greater interannual variability is found in the more recent (1994–2000) analyses than in the earlier (1979–1993) reanalyses.